Filter darkroom light



Oct. 8, 1968 FIGD 4 Fna-6 J. J. DOLAN ET Al- FILTER DARKROOM LIGHT Filedoct. s, 196e INI/ENTORS JAMES J. DOLAN DAVID J. DEGENKOLB mwa ga/JtwATTORNEYS United States Patent O 3,405,262 FILTER DARKROM LIGHT James J.Dolan, Woodland Hills, and David J. Degenkolb, Mission Hills, Calif.,assignors to Texrtron Electronics, Inc., Sylmar, Calif., a corporationof Delaware Filed Oct. 3, 1966, Ser. No. 583,605 6 Claims. (Cl. 240-20)ABSTRACT F THE DISCLOSURE A safe light for photographic darkrooms isprovided in the form of a double walled cylindrical holding structureincorporating a light source, filter, and a honeycomb structure definingVstraight parallel passages forv limiting the angle of divergence of theoutput light. The filter includes a first piece of glass having amulti-layer dielectric coating and a dichroic filter positioned on theopposite side of the multi-layer dielectric coating so that the coatingis sandwiched between the glass and dichroic filter. The coatingreflects light radiation falling outside a given spectral pass band forwhich film being Iprocessed is relatively insensitive. Thus a largedegree of illumination is provided for working on the film with minimumrisk of damaging the film itself.

This invention relates to an improved safe light primarily useful indarkrooms and film processing plants.

It is well known that many black and white and color photographic filmsexhibit low sensitivity responses at certain spectral wave lengths.Illuminating means for providing sufficient light for workers during theprocessing or developing of such film should accordingly be designed toemit light of radiation having wave lengths to which the film isrelatively insensitive. In this respect, a sodium vapor type lamp couldvery effectively be used in darkroom applications as a suitable safelight since many photographic films have a low sensitivity response atthe spectral frequency of the sodiumv lines. However, the initialinvestment in a sodium vapor lamp and the relatively short lifetime ofthe bulb makes its use in many instances unsatisfactory from aneconomical standpoint.

An alternative to the use of a relatively expensive sodium lamp would beto provide a conventional tungsten type lamp in combination with sometype of absorbing medium to block light radiations harmful to the film.In this respect, there has heretofore been provided one type of safelight including an absorbing type filter. However, the pass band of thisfilter is defined by employing high absorption materials with the resultthat a broad pass band results that can well overlap into sensitivespectral regions of the film. In addition, the light intensity isrelatively low, usually less than If a high efiicient multi-layerdielectric filter should be used with a conventional lamp, a problemarises in that any defined pass band tends to shift as a function of theangle of incidence of the light from the light source striking thefilter. The end result is again a possible overlapping of the pass bandwith sensitive spectral regions of the film.

With the above in mind, it is a primary object of the present inventionto provide a vastly improved safe light particularly useful inphotographic darkrooms in which the foregoing problems are overcome.

More particularly, it is an object to provide a safe light whichprovides a substantially higher degree of light intensity than hasheretofore been available without the risk of damaging the film beingprocessed.

Other objects are to provide a high intensity safe light which issubstantially less expensive to maintain than presently available safelights of comparable 'efficiency 3,405,262 Patented Oct. 8, 1968 and isof substantially longer life thereby minimizing replacement expenses.

Briefiy, these and many other objects and advantages of this inventionare attained by combining a conventional light source with a filtermeans positioned relative to the source to filter light from the source.The filter means defines a narrow spectral pass band of hightransmission for light radiation of wave lengths in the pass band andnegligible transmission for light radiation of wave lengths outside thepass band. The spectral pass band itself falls within a band of lightradiation to which most photographic film is substantially insensitive.In addition, absorption type filtering is not utilized in the band passregion so that high intensity radiation is passed by the filter and ahigh degree of illumination is available to the worker.

To avoid shifting of the spectral pass band as a consequence of lightpassing through the filter at angles of incidence deviating from thenormal angle of incidence, the safe light, in its preferred embodiment,further includes a light angle limiting means. This means functions tolimit the angle of light passing from the safe light structure withrespect to the axis of the light beam, to a value less than a givenangle.

A better understanding of the invention will be had by now referring toa preferred embodiment thereof as illustrated in the accompanyingdrawings, in which:

FIGURE l is a rear perspective view of the safe light;

FIGURE 2 is a front perspective view;

FIGURE 3 is an enlarged cross section in the direction of the arrows 3 3of FIGURE 2 showing some of the basic components making up the safelight with other components in full lines;

FIGURE 4 is a highly diagrammatic illustration of the multi-layerdielectric filter means used in the structure of FIGURE 3;

FIGURE 5 is a curve of the transmission characteristics over variouswave lengths of the filter in FIGURE 4 superimposed on a filmsensitivity characteristic Plot and on a prior art filter curve; and,

FIGURE 6 is a diagram useful in explaining the transmissioncharacteristics of the filter as a function of the angle of incidence oflight on the filter.

Referring first to FIGURE l, the safe light comprises a holdingstructure in the form of coaxially positioned outer and innercylindrical casings 10 and 11. As shown, there is provided a lead inwire 12 for connectlng a source of electrical energy such asconventional volt 60 cycle power to a suitable lamp within the innercasing 11. Also illustrated in FIGURE 1 is a switch 13 for turning thesafe light on and ofir and a rheostat 14 for adjusting the intensity ofthe light.

As will be evident from both FIGURES l and 2, light from the safe lightpasses from the opposite end of the housing as indicated at 15. Heatgenerated by the light source in the housing is dissipated by theprovision of suitable air holes in the inner casing 11, such asindicated at 16 and 17. Also visible in FIGURE 2 is a light anglelimiting means in the form of a honeycomb structure 18 defining .aplurality of longitudinal passages in the front portion of the holdingstructure. Suitable screws 19 and 20 hold the casings in coaxialrelationship to each other.

Referring now to the detailed cross section of FIGURE 3, it will benoted that the screws 19 cooperate with an annular spacing ring 21 tohold the outer casing 10 in proper coaxial, spaced relationship withrespect to the inner casing 11 to define an annular air spacetherebetween. The screws 20 also cooperate in holding the other end ofthe outer casing 10 in proper, coaxially spaced relationship relative tothe inner casing land also permit communication of the annular spacebetween the casings with the atmosphere.

The inner casing 11 includes additional air holes 22 and 23circumferentially spaced about the casing for permitting air circulationto the annular space between the casings and thus to the atmosphere.

A light source in the form of a conventional bulb such as a tungstenbulb 24 is incorporated in the inner casing 11 and supported in asuitable socket 25 mounted on a supporting plate 26. The supportingplate 26 includes air holes 27 and cooperates with a second plate 28having air holes 29 to provide la light partition. In this respect, theholes 27 are staggered with respect to the holes 29, and these latterholes are circumferentially staggered relative to the holes 16 and 17 inthe extreme rear of the inner casing so that proper air circulation cantake place without any straight line pass being defined to the exteriorof the holding structure. As a consequence, light is blocked frompassing out through these holes.

In a similar manner, the positioning of the holes 22 and 23 in the innercasing 11 are such that any light passing through these holes will beblocked by the surrounding outer casing A filter means 30 is supportedin the inner casing on a mounting ring 31 in front of the lamp 24. Thelight angle limiting means 18 described in FIGURE 2, in turn, isincorporated in the inner casing towards the front end of the casing forreceiving light from the filter 30. The plurality of longitudinallyextending str-aight, parallel passages defined by the honeycombstructure 18 are shown at 18. It will be evident that light passing intoeach passage at an angle greater than a given angle will be blocked frompassing through the passage. The structure 18 thus serves in one respectas a light collilnating means.

Referring now to FIGURE 4, a preferred type of filter making up thefilter means 30 described in FIGURE 3 is shown in detail. Basically thefilter means includes a dichroic filter 32 which may comprise nineteenalternate layers of dielectric material coated on a suitable glasssubstrate. Following this dichroic filter is a second glass substrateupon which are coated alternate layers of dielectric material indicatedgenerally at 33. In the particular filter shown, these layersconstitute, respectively, quarter wave length thick coatings of zincsulfide and sodium aluminum fiuoride, commonly referred to as cryolite.The quarter wave length zinc layers are indicated as )t/4Z lwhile thecryolite layers are indicated by 4 K.

It will be noted that a total of six layers are Provided on thesubstrate end glass of the dichroic filter 32. Following these sixlayers is a spacer layer 34 of zinc sulfide, preferably six-fourths of awave length in thickness `as indicated. This spacer layer, as is wellknown to those skilled in the art, controls the width of the pass bandof the overall filter.

Following the spacer layer 34 are further alternate layers of zincsulfide and cryolite as indicated a-t 35. Eleven such layers are shownfollowed by another spacer layer 36 of zinc sulfide of 4/ 4 Wave lengththickness. Finally, there are provided six alternate layers of zincsulfide and cryolite following the spacer layer 36. The opposite or leftend of the filter as illustrated in FIGURE 4 terminates in an absorbingmedium 38, preferably a piece of glass such as commercially availableCorning 3482. This glass is impregnated with a suitable dye designed toabsorb Iradiation of wave lengths shorter than 560 millimicrons.

Light from the bulb 24 of FIGURE 3 first passes through the dichroicfilter 32 and then through the multilayer filter to exit as at from theabsorbing glass piece 38. Essentially, the alternate layers ofdielectric material constituting the multi-layer coating, refiectradiations of wave lengths on either side of a defined spectral passband.

With particular reference now to FIGURE 5, there is illustrated by thesolid lines 40 the film sensitivity over a range of wave lengths of aclass of photographic films with which the safe light of this inventionmay be used. Also illustrated is a dotted curve 41 indicative of priorart type absorption medium used in conjunction with a conventional lampfor safe light purposes, and a dashed line curve 42 defining the bandpass characteristics of the filter of this invention for comparisonpurposes.

With respect to the prior art absorption characteristics as indicated bythe dotted curve 41, it will be evident that any pass band involved isrelatively broad and that the transmission of radiation to which thefilm is relatively insensitive is less than 10%. It will also be clearthat as a consequence of the broad width and shallowness of this passband characteristic, there is overlapping onto the film sensitiveregions defined by the solid curves 40 on either side of this pass band.

On the other hand, it will be clear that the extremely narrow pass bandcharacteristic of the filter described in FIGURE 4 and as indicated bythe dashed curve 42 provides for well over 50% transmission forradiation of wave lengths falling outside the sensitive wave lengthsdefined by the solid curve 40. The pass band 42 has relatively sharp cutoff characteristics so that there is substantially no seriousoverlapping of the film sensitive curves.

From the curve 42, it will be evident that radiation of wave lengthsbetween approximately 580 and 600 millimicrons will be transmitted bythe filter and the radiations of wave lengths between 589 and 592millimicrons will be passed to an extent considerably greater than 50%.Transmission of radiations of wave lengths lying outside these values,on the other hand, is considerably less than 10%.

As a consequence, it will be clear that a relatively high intensitylight is provided without risk of damaging the film so that a markedimprovement is provided over any prior art type of safe light presentlyavailable.

Referring now to FIGURE 6, there is illustrated a series of straightlines 39 angulating from a single point to the right. The linedesignated 0 corresponds to light radiation striking the filter at anormal angle of incidence; that is, the filter surface is perpendicularto the axis of travel of the light. For the particular filter describedin FIGURES 4 and 5, there will be provided substantially 72%transmission of radiation of wave lengths in the neighborhood of 5905 A.However, for light striking at an angle of incidence of 10", it will benoted that 70% transmission results for radiations of wave lengths inthe neighborhood of 5800 A. units. For incidence angles of 20 and 30,the transmission is progressively less as shown and it will be notedthat the wave lengths transmitted, in turn, become progressivelyshorter.

In essence, as the angle of incidence increases, the spectral pass bandshifts towards the shorter wave lengths. As a consequence, and againwith reference to FIGURE 5, this shifting of the spectral pass bandtowards the shorter wave lengths can result in possible overlapping ofthe film sensitivity spectral regions indicated by the curves 40. l

In accordance with a feature of this invention it is desirable as aresult of the foregoing phenomenon to limit the angle of light passingfrom the housing structure described in FIGURE 3 to an angle relative tothe light axis or axis of the housing structure of 10 or less. Thisangle limitation is effected by the several straight line passagesdefined by the honeycomb structure 18 as described heretofore inconjunction with FIGURE 3.

From the foregoing description of the safe light of this invention, itsoperation will be evident.

With the wires 12 connected to a suitable power outlet, the lamp 24 isenergized by closing the switch 13. The degree of illumination may bereadily controlled by the rheostat 14. The housing structure itself isoriented so as to direct the light beam 15 onto various items or filmwith which a Worker or film processor may be dealing. As a result of thefiltering characteristics described, there will be a relatively highdegree of illumination without fear of damaging any film beingprocessed.

Heat generated by the lamp is properly dissipated by convection aircurrents passing through the various air holes described. By providingthe double casing structures and 11, there are defined suitable airpassages for proper air circulation without in any way permitting lightfrom the lamp 24 from escaping through the casing structures other thanout the front wherein its angle from the desired directional axis of thelight is limited by the honeycomb structure 18. In addition, the doublecasing structure permits handling of the safe light without the operatorburning himself inadvertently, the outer casing 10 remainingconsiderably cooler than the inner casing.

Should the light bulb burn out, the bulb itself may be easily andinexpensively replaced with a similar commercially available tungstenbulb. The bulb itself is the only major element subject to maintenance,the filter and light angle limiting means constituting passivecomponents in the structure.

From the foregoing description, it will be evident that the presentinvention has provided a greatly improved safe light particularly usefulin photographic darkrooms and other film processing plants wherein allof the various objects set forth heretofore are fully realized.

While the preferred embodiment of the invention has been described withrespect to film processing, it will be evident that the safe light maybe used in other applications involving items sensitive to certain lightradiations. The invention accordingly is not to be thought of as limitedto the specific embodiment set forth for illustrative purposes.

What is claimed is:

1. A safe light comprising, in combination: a holding structure; a light-source mounted in said holding structure; a filter means positioned insaid holding structure to filter light from said source such that lightfrom said filter means is predominantly defined by a given spectral passband, said filter means including at least one piece of glass having amulti-layer dielectric coating, and a dichroic filter positioned on theopposite side of said multi-layer dielectric coating so that saidcoating is sandwiched between said glass and dichroic filter forreflecting light radiation falling outside said given spectral pass bandto thereby define said pass band; and light angle llimiting meanspositioned in said holding structure to limit the angle at which lightcan pass from said holding structure to a given angle whereby said safelight may be used to illuminate safely items predominantly sensitive tolight radiation falling outside said given spectral pass band andrelatively insensitive to radiations falling within said lspectral passband.

2. A safe light according to claim 1, in which said holding structurecomprises outer and inner casings in coaxial relationship to define anair space between the inner wall of said outer casing and the outer wallof said inner casing, said light source being mounted in said innercasing, said inner casing including air holes communicating with saidair space to permit air circulation to said air space, said air spacecommunicating with the exterior atmosphere at a point positionedrelative to said air holes such that light from said light sourcepassing through said air holes is blocked by said outer casing.

3. A safe light according -to claim 1, in which said light anglelimiting means comprises a plurality of straight, parallel, passages ofgiven longitudinal extent such that light entering one end of eachpassage at an angle to the longitudinal axis of said passage greaterthan said given angle, is blocked by said passage `from passingtherethrough.

4. A safe light according to claim 3, in which said given angle isapproximately 10.

5. A safe light according to claim 1, in which said glass in said filtermeans exhibits a relatively high absorption cross section for lightradiation of wave lengths shorter than the wave lengths in said givenband.

6. A safe light according to claim 1, in which said pass band includeslight radiations of wave lengths between 580 and 600 millimicrons, saiditems constituting photographic film to be processed.

References Cited UNITED STATES PATENTS 1,379,949 5/1921 Wedmark240-46.39 2,412,496 12/ 1946 Dimmick 350-166 2,545,274 3/1951 Golden240-20 2,742,819 4/1956 Koch et al. 350--166 2,799,773 7/ 1957 Schwartz240-47 OTHER REFERENCES M. Banning, Practical Methods of Making & UsingMultilayer Filters, October 1947, Journal of Optical Soc. of Amer. pp.792 to 797.

NORTON ANSHER, Primary Examiner.

RICHAR-D M. SHEER, Assistant Examiner.

